docs: def photbw(self, floor=0):
          """Calculate :ref:`pysynphot-formula-bandw`.
  
          .. note:: For backward-compatibility with IRAF STSDAS SYNPHOT only.
  
          Parameters
          ----------
          floor : float
              Same as :meth:`rmswidth`.
  
          Returns
          -------
          ans : float
              RMS band width (deprecated).
  
          """
          mywaveunits = self.waveunits.name
          self.convert('angstroms')
  
          wave = self.wave
          thru = self.throughput
          self.convert(mywaveunits)
  
          # calculate the average wavelength
          num = self.trapezoidIntegration(wave, thru * N.log(wave) / wave)
          den = self.trapezoidIntegration(wave, thru / wave)
  
          if num == 0 or den == 0:
              return 0.0
  
          avg_wave = N.exp(num/den)
  
          if floor != 0:
              idx = N.where(thru >= floor)
              wave = wave[idx]
              thru = thru[idx]
  
          # calcualte the rms width
          integrand = thru * N.log(wave / avg_wave)**2 / wave
          num = self.trapezoidIntegration(wave, integrand)
  
          if num == 0 or den == 0:
              return 0.0
  
          return avg_wave * N.sqrt(num/den)